The present invention relates to a process for forming microcapsules.
Microcapsules containing solid, liquid and gaseous core materials have been commercially utilized in various applications. One of the most widespread uses of such microcapsules has been in the art of pressure-sensitive copy systems where minute droplets of an oily material containing a chromogenic compound are encapsulated and coated onto an imaging transfer sheet. Upon rupture of the microcapsules under localized pressure, the chromogenic material is transferred by means of the oily vehicle to an underlying developer sheet having an absorbent coating of developer material which reacts with the chromogenic material to produce a visible colored image at points corresponding to where the microcapsules have been ruptured and the chromogenic compound transferred.
Many combinations of materials have been used in the past in search of compositions which yield certain physical characteristics in capsule walls or which permit performing the encapsulation process under certain desired or required conditions. As examples of desirable capsule characteristics, small size, impermeability of capsule walls to diffusion, and strength of capsules walls to withstand normal handling forces are common. As examples of desirable process conditions, relatively short times and relatively high yield and concentration are important.
Various processes have been proposed for producing microcapsules including both chemical and physical phenomena. For example, U.S. Pat. Nos. 4,460,722 and 4,557,755 disclose a process wherein a water-soluble cationic urea resin and a prepolymer composed of formaldehyde and urea or a melamine are condensed in the presence of an anionic surfactant to form the microcapsule wall. U.S. Pat. No. 4,328,119 discloses a process wherein an anion-modified aminoaldehyde resin is polycondensed in the presence of at least one anionic colloid material to form the microcapsule wall. Other processes for forming microcapsule walls are disclosed in U.S. Pat. Nos. 4,251,386; 4,423,091; 4,444,699; and 4,450,123.
Das Papier, 39, 12, 601 (1985) discloses a process for the production of microcapsules having a diameter of about 200-5000 microns by dripping an aqueous solution of the core material and cellulose sulfate into an aqueous solution of a polymer cation.
As a preface to the present process, U.S. Pat. Nos. 4,452,957; 4,455,408; and 4,517,351 teach the reaction of quaternary ammonium monomers and monomers copolymerizable therewith in the presence of anionic polymers to directly form a dry coacervate. The dry coacervate is ground to make a powder useable as electroconductive agents in photographic, fiber, membrane or paper applications. More specifically, the process involves forming a solution of the monomers, the anionic polymer, solvent and a free-radical initiator; polymerizing and drying the solutions at a temperature above the solvent's boiling point; and recovering the dry product. The theory behind the polymerization of the quaternary ammonium monomers and the anionic polymer applies to the present invention.